The present invention relates to coated cutting tool inserts particularly useful for machining of nodular cast iron parts by turning at high speeds.
Cast iron materials may be divided into two main categories, grey cast iron and nodular cast iron. Nodular cast iron is considered to be a comparatively difficult material to machine. Cast irons often have an outer layer of cast skin, which may contain various inclusions of sand, rust and other impurities, and also have a surface zone which is decarburized and contains a higher amount of ferrite than the rest of the material. In addition, machining of nodular cast iron causes so called adhesive wear to Al2O3-coated cutting tools as well as higher levels of abrasive wear. Adhesive wear occurs when fragments or individual grains of the coating layer are pulled away from the cutting edge by the work piece chip formed. Flaking of this kind, close to the edge line, eventually results in a local break through of the coating layer leading to an accelerated chemical wear and shorter tool life.
Another critical factor when machining nodular cast iron at high speeds with or without the use of coolant is the formation of excess heat at the cutting edge causing a softening of the inserts leading to plastic deformation and flaking of the coating with accelerated wear as a consequence.
U.S. Pat. No. 5,863,640 discloses a coated cutting insert consisting of a cemented carbide body of a composition 5-11 wt-% Co, <10 wt-% cubic carbides of the metals Ti, Ta and/or Nb and balance WC, and a coating comprising an innermost 0.1-2 μm TiCxNyOz layer, a 2-15 μm TiCxNyOz layer, a 0.1-2 μm TiCxNyOz layer, a 2-10 μm α-Al2O3 layer, especially suited for machining of low alloyed steel components by turning.
U.S. Pat. No. 5,945,207 discloses a coated cutting tool insert with a WC—Co based substrate with <0.5 wt-% cubic carbides and a coating comprising an innermost 0.1-2 μm TiCxNyOz layer, a 5-10 μm TiCxNy layer, a 0.1-2 μm TiCxNyOz layer, a 3-6 μm α-Al2O3 layer, and an outer layer of TiCxNyOz a 0.5-3 μm layer or a multilayer of a 0.5-3 μm thick TiN+TiC+TiN sequence, particularly useful for machining of cast iron.